Q1: Let S be the region in the first quadrant bounded by the curves xy=1, xy=3, x2 - y2 = 1, and x2 - y2 = 4. Compute ∫∫(x2 + y2)dA. S (Hint: Let G(x,y)=(xy, x2 - y2). What is |det DG|?) Solution: I don't understand the third and fourth equalities in the solutions. Can someone kindly explain?